SIP method subscribe

When a SUBSCRIBE request is sent to a PINT Server, it indicates that a user wishes to receive information about the status of a service session. The request identifies the session of interest by including the original session description along with the request, using the SDP global-session-id that forms part of the origin-field to identify the service session uniquely.

The SUBSCRIBE request (like any other SIP request about an ongoing session) is sent to the same server as was sent the original INVITE, or to a server which was specified in the SIP header contact: field within a subsequent response (this might well be the PINT gateway for the session).

In principle, a user might send a SUBSCRIBE request after the telephone network service has completed. This allows, for example, checking up "the morning after" to see if the fax was successfully transmitted. However, a PINT gateway is only required to keep state about a call for as long as it indicated previously in an SIP header expires: header sent within the response to the original INVITE message that triggered the service session, within the response to the SUBSCRIBE message, within the response to any UNSUBSCRIBE message, or within its own UNSUBSCRIBE message.

The RFC for SUBSCRIBE doesn't really define any applications. The applications are called "Event packages" and are registred with the IANA. Current event packages could is listed here: